What causes guys to have a curved penis, instead of a straight one ?

There are three sponge like tissues in the penis that fill with blood upon erection, one on the left, another on the right and one on the bottom.

If one or more of these is smaller or bigger than the other(s), it can cause the penis to be slightly curved in one direction or another.

Since most humans aren't symmetrical, having a curve in the penis is rather common.

An extreme curve which may be painful and/or prohibits normal sexual use of the penis is known as Peyronie's Disease and one should consider seeing a Urologist to diagnose what can be done.

Genetics, almost entirely. Not really any other reason a guy's penis would curve.

Well there could be a number of different things that cause it however its common for an erection to have a curve or bend to it. NORMAL.

Genetics; but sometimes, you can get a bit of curvature if you masturbate with a too-firm grip in one direction.

Genetics , they haven't done anything to damage their penor

My poon muscles. They can twist your D up like a twist tie.